In this note, we study umkehr maps in generalized (co)homology theories arising from the Pontrjagin-Thom construction,
from integrating along fibers, pushforward homomorphisms, and other similar constructions. We consider the basic
properties of these constructions and develop axioms which any umkehr homomorphism must satisfy. We use a
version of Brown representability to show that these axioms completely characterize these homomorphisms, and a
resulting uniqueness theorem follows. Finally, motivated by constructions in string topology, we extend this axiomatic
treatment of umkehr homomorphisms to a fiberwise setting.